A. Technical Field
The present invention relates generally to metal processes and in particular to apparatus and method for shrouding a molten metal stream during a teeming, or molten metal pouring, operation.
B. Description of the Prior Art
In the making of metal, such as steel, refined molten metal is poured from a ladle to one or more ingot casting molds Each mold consisting of a bottom and an upright rectangular tubular wall section, can hold several (e.g. 15 to 24) tons of molten metal. After transferring the molten metal to a mold by pouring, the molten metal and the mold are allowed to cool, after which a solid ingot of metal is removed from the mold by lifting off the wall section The ingot is of a size and configuration which facilitates subsequent processing, or working, by other machinery into forms, such as rolled sheets, from which the metal can be further worked and/or cut to produce pieces of metal from which products can be made.
During a pouring, or "teeming" operation, a ladle, holding about 200 tons of molten metal and having at least one nozzle extending downwardly from its base or bottom, is moved by an overhead crane into a position with the nozzle aligned above the upper, or open, end of the mold, and then lowered into pouring position. A valve associated with the nozzle is opened to allow the molten metal to exit the bottom of the ladle by gravity and enter the mold. The molten metal thus fills the mold. When the mold is completely filled, the valve is closed to terminate the flow of molten metal out of the ladle. The ladle is then, sometimes after being raised up away from the top of the mold, translated a distance to align the nozzle with the top of the next mold to be filled. The ladle is then (if it was raised) relowered to position the nozzle near the top of the mold.
In instances in which the molten metal is "killed" steel, (i.e. completely deoxidized) it is imperative for obtaining high quality steel that the molten metal stream leaving the ladle nozzle be isolated or shrouded from atmospheric air as much as possible, in order to prevent re-oxidation of the metal. If the molten metal stream is exposed to the atmosphere, oxygen is drawn into the stream and produces reoxidation products, often termed "inclusions" in the cast material which lower the overall quality of the cast. Of particular concern are alumina inclusions, which constitute an extremely hard substance which interferes with subsequent working of the steel. Many applications require high grade steel and it has been found that steel produced by prior art methods, in which the stream was poorly shrouded, was unacceptable.
In some applications, an inert gas such as argon is injected in the region around the lower portion of the molten stream to inhibit atmospheric air from contacting the stream. Also, inert gas is sometimes introduced into the lower portion of the mold as well. In some cases, chemical compounds, such as paraffin, are placed in the mold before pouring. These compounds react in the presence of the molten steel to emit gases which are intended to perform the same function as those gases introduced from the outside, such as the argon described above.
Sometimes, a perforated ring was provided extending about the molten stream and connected to a source of natural gas. The natural gas was discharged into the region of the lower portion of the stream and ignited. It was thought that the burning would reduce the amount of oxygen in the ambient atmosphere around the stream and thereby reduce re-oxidation. In practice, this procedure did little good.
Inert gases such as argon can be very expensive and it is important that the gas be confined as well as possible around the stream
Attempts have been made to shroud the molten metal stream as it travels from the ladle into the mold. Other methods have been proposed for shrouding the interface between the ladle and the mold.
One such shrouding apparatus is described in U.S. Pat. No 4,589,465, issued on May 20, 1986, to Gerding, et al. and entitled "Top Pour Shroud", which patent is hereby expressly incorporated by reference.
The Gerding device included a steel plenum box affixed to the bottom of the ladle and surrounding the nozzle. When the nozzle was opened, the molten steel would drop by gravity through the box, essentially without touching it, and be deposited in the mold. The Gerding device also included a heavy steel hood defining a hole aligned with and forming a rough seal about the plenum box and through which the box could protrude. The hood was also coupled to the bottom of the ladle via a power operated mechanism which could raise and lower the cover plate with respect to the bottom of the ladle. When it was desired to top pour an ingot mold, the ladle, with the associated shrouding structure, would be moved such that the plenum box and nozzle were aligned with the central portion of the top opening of the ingot mold. The hood would then be lowered until it contacted the upper rim of the open top of the mold, providing another rough seal to keep the inert gas within the mold and within the plenum box with minimum leakage to the outside.
In order to facilitate this sealing function, the hood in the Gerding device was pivotable about only a single horizontal axis, and movable vertically. These degrees of freedom assisted in enabling the cover plate to better approximate a better seal about the top end of the mold than would be the case if it did not have this versatility.
While this apparatus was useful, particularly for top pouring small ingots, it exhibited significant problems when used in pouring larger ingots. For one thing, the Gerding apparatus was very heavy, expensive and complex. Such as, it was difficult to install on and remove from the ladle. Using this prior art arrangement, set-up and take-down time for the device was on the order of 20 to 25 minutes each way, and required two to three men, as well as the services of a crane and operator.
Set-up and take-down of the shrouding apparatus is necessary before and after each operation in which a filled ladle is completely emptied into a succession of about 8 ingot molds, the whole operation being termed a "pour". Typically, it is desired to achieve about several dozen "pours" per day. It can thus be seen that the time lost setting up and taking down the prior art apparatus significantly raises the cost of producing the steel.
Another problem with the prior art apparatus was that the hood, even though having two degrees of rotational freedom, sometimes did not effect a satisfactory degree of seal with the upper rim of the mold. This condition obtained because usually the upper rim of the mold is not completely level. Complicating the situation is the additional fact that the upper rim of the mold tends to become encrusted with an uneven residual deposit of cooled solid metal which forms a rough surface The hood of the prior art apparatus was not, often, sufficiently compliant to effect the quality or degree of sealing necessary for good shrouding.
Another technique for transferring molten metal from a ladle into an ingot mold is called "bottom pouring". In bottom pouring, a tall vertical conduit, sometimes called a "trumpet", communicates at a lower end with the bottom of several ingot molds. The trumpet extends upwardly beside the mold to an elevation somewhat higher than that of the upper rim of the mold. The ladle is positioned with its nozzle aligned with the upper end of the trumpet When the nozzle is opened and molten steel flows into the trumpet, the level of molten metal in the molds rises from the bottom, exposing only the upper surface of the rising column of molten metal to whatever gas may be present in the mold. There is less agitation and splashing of molten metal than in the case of top pouring, which generally reduces the vulnerability of the steel to degradation by contact with atmospheric gases.
While bottom pouring is a worthwhile technique to use where specifications call for very high purity steel, and a high degree of freedom from inclusions, bottom pouring is not considered advantageous for producing most grades of steel, due to its far higher cost when compared with top pouring It is estimated that, to accomplish one pour by bottom pouring techniques, costs several thousands of dollars more than top pouring without a shroud. The cost of top pouring an equivalent amount of steel is far less. For this reason, bottom pouring has not generally been used for any applications other than making extremely high quality, inclusion free and expensive steel.
It is an object of the present invention to provide a technique and apparatus for producing steel having a level of purity and freedom from inclusions rivaling that of the bottom pouring method, by the use of the much less expensive top pouring of ingot molds.